Before us is Mishnah 6 in the seventh chapter of Maseches Shevi'is, which deals with establishing the times of bi'ur for grasses and leaves, whether they were gathered while moist or gathered while dry.
One who gathers moist grasses:
The Mishnah opens: "HaMelaket asavim lachim - ad sheyivash hamatok" - one who gathers moist grasses. A person who gathered moist grasses that are fit for eating may eat from them until the time of bi'ur, and their time of bi'ur is "until the sweetness dries up." What is that sweetness? It refers to the moisture in the ground: once that moisture has dried up - and this was a particular point in the season that was familiar to them - a person knows that the time of bi'ur has arrived for moist grasses that are fit for eating, and from that point on he is obligated to remove them.
One who gathers from the dry:
For one who gathers dry grasses, their time of bi'ur is "ad shetered revi'ah sheniyah" - until the second rainfall comes down.
What is the "second rainfall"?
In Eretz Yisrael there are three rainfall periods in the autumn, and the Tanna'im in the Mishnah in Maseches Ta'anis disagree about exactly when they occur:
Rabbi Meir: the third of Cheshvan, the seventh of that month, and the twenty third of it. According to him the second rainfall, the middle one, is on the seventh of Marcheshvan.
Rabbi Yehudah: the seventh, the seventeenth and the twenty third. According to him the middle one is on the seventeenth and not on the seventh.
Rabbi Yosi: the seventeenth, the twenty third and Rosh Chodesh Kislev. According to him the middle one is on the twenty third.
Our Mishnah works out according to each of these opinions, since all of them are speaking about that same middle rainfall. Until that point a person may make use of the dry grasses for eating.
Reed leaves and vine leaves:
"V'alei kanim v'alei gefanim - ad sheyishru me'ivyan" - and reed leaves and vine leaves, until they fall from their stalks. For reed leaves and the leaves of grapevines, their time of bi'ur is when they drop off their stalks. And one who gathers these leaves when they are already dry, their time of bi'ur arrives with the coming down of the second rainfall, as stated above.
The opinion of Rabbi Akiva:
"Rabbi Akiva omer: b'chulan ad shetered revi'ah sheniyah" - Rabbi Akiva says: for all of them, until the second rainfall comes down. For everything mentioned in our Mishnah - the moist grasses, the dry grasses and the leaves, whether moist or dry - the time of bi'ur is one and the same, at the coming down of the second rainfall in the autumn.
In summary: in this Mishnah we learned the times of bi'ur for grasses and leaves: for moist grasses, "until the sweetness dries up," which is the moisture in the ground; for dry grasses and dry leaves, until the second rainfall comes down; and for reed leaves and vine leaves, until they fall from their stalks. We clarified the identity of the second rainfall according to the opinions of Rabbi Meir, Rabbi Yehudah and Rabbi Yosi, and we saw that Rabbi Akiva sets a single time for all of them: the coming down of the second rainfall.